How To Setup Sidebar So It Stops Floating

Hello,

I was not even sure how to ask this in the title.

Here is what I am trying to do, but if it requires lots of coding, I might skip it.

I am trying to lock the scrolling action of the sidebar so that when the viewer reaches the bottom of the content on the sidebar, it locks the sidebar content in place (on the screen) and allows the main content to continue to scroll.

There is a good example of this here: http://expertvagabond.com/gopro-travel-accessories/. Notice a section of the sidebar “locks” when you reach the bottom of the sidebar content.

Thanks for any suggestions!
Russ